I. Field of the Invention:
This invention relates generally to a cardiac pacing system in which the pacing rate tracks metabolic need, and more particularly to a rate adaptive pacemaker employing a sensor for detecting variations in cardiac chamber volume or pressure due to respiration and producing a control signal related to the respiration frequency and depth for adjusting the pacing rate relative to a baseline value.
II. Discussion of the Prior Art:
Workers in the cardiac pacing field have long recognized the desirability of creating an implantable device capable of maintaining an adequate heartrate in a patient suffering from bradycardia at a nominal level with the individual at rest but which would automatically adapt to changes in metabolic need to increase the pacing rate above that baseline value. One of the earliest attempts at providing such a rate adaptive pacemaker is set forth in the 1977 U.S. Pat. No. 4,009,721 to Mario Alcidi. Alcidi describes a variable rate pacer in which a sensor is provided for measuring blood Ph and developing a control signal proportional to that factor, recognizing that the blood becomes more acidic with exercise. Because of the difficulty in obtaining a reliable sensor which would not drift with time, the Alcidi device never became commercially successful.
The patent literature is replete with rate adaptive pacemaker designs in which a wide variety of physiologic parameters are sensed as an indicator of metabolic need and used to develop a pacing rate control signal for a rate adaptive pacemaker. The following table summarizes several of these approaches:
______________________________________ INVENTOR U.S. Pat. No. CONDITION SENSED ______________________________________ Krasner 3,593,718 Respiration rate Dahl 4,140,132 Physical activity/motion Witzfeld, et al 4,202,339 Blood oxygen saturation Rickards 4,228,803 QT interval in an ECG waveform Knudson, et al 4,313,442 Change in atrial rate Cook, et al 4,543,954 Venous blood temperature Koning, et al 4,566,456 Right ventricular systolic pressure Plicchi, et al 4,596,251 Minute ventilation (respiration) Salo, et al 4,686,987 Stroke volume Nappholz, et al 4,702,253 Minute volume (respiration) Thornander, et al 4,712,555 ECG measured interval Koning, et al 4,716,887 Right ventricular blood pCO.sub.2 Chirife 4,719,921 Pre-ejection period Amundson 4,722,342 Multiple difference sensors Koning 4,730,619 Ejection time Callaghan 4,766,900 Change in depolarization gradient magnitude Citak, et al 4,773,401 Pre-ejection interval Elmquist, et al 4,790,318 Respiration Lekholm 4,817,606 Myoelectrical signals ______________________________________
The above list is not intended to be exhaustive in that various other workers have received U.S. patents on rate adaptive pacemaker devices and improvements utilizing the sensed parameters set forth in the table and are considered cumulative.
In our earlier U.S. Pat. No. 4,686,987, there is described a rate adaptive cardiac pacer in which means are provided for measuring stroke volume by utilizing an intracardiac impedance waveform and deriving stroke volume-related information from the peak-to-peak swings in the measured impedance waveform. A closer analysis of the intracardiac impedance waveforms reveals amplitude variations due to changes in intrathoracic pressure. In particular, in that atrial and ventricular volumes are affected by factors which influence preload (i.e., the filling of the chamber) and after-load (i.e., the resistance to flow out of the chamber) and that these factors are modulated by the intrathoracic pressure, it becomes possible to monitor variations in intrathoracic pressure by following the low frequency variations in cardiac volume or pressure parameters. For example, the beat-by-beat variation in end-diastolic volume, end-systolic volume, average ventricular volume, or stroke volume or correspondingly, the end-diastolic pressure, end-systolic pressure, average pressure or pulse-pressure may be used as indicators of intrathoracic pressure.
Because the intrathoracic pressure is directly related to respiration (pressure drops during inspiration and increases during expiration), the amplitude of the variation in intrathoracic pressure during a respiratory cycle is directly related to the depth of respiration.